Conventional electrophotographic printers are constructed with an image fixing station spaced downstream from an image transfer station. Print media, such as paper, is moved through the image transfer station and then through the image fixing station. At the image transfer station, an electrophotographic image is transferred to the print media. At the image fixing station, the image is affixed to the print media. The print media is moved through the stations by a media motion assembly which typically includes a roller assembly located at the image fixing station, a guidance system positioned intermediate of the image transfer station and the image fixing station, and a motor connected to drive the roller assembly and the guidance system.
To form an electrophotographic image on a print media, an image forming device at the image transfer station places multiple scan lines at a preset scan rate on a cylindrical photoconductive drum. A series of scan lines forms the desired image on the drum. The drum is then rotated adjacent the print media to transfer the image to the print media, which is moving through the image transfer station and past the drum at a constant velocity. The media motion assembly ensures that the media moves at a constant velocity; otherwise, image quality is severely diminished.
The velocity of the media through the image transfer station and the scan rate of the image forming device are independent of each other. However, for optimum printing quality, the media velocity and scan rate must be synchronized. For example, in one electrophotographic printer, the optimum printing synchronization may be to provide 300 scan lines per inch of print media. Accordingly, the media motion assembly must move the print media past the image transfer station at a velocity which permits the drum to transfer 300 scan lines per inch to the print media.
Electrophotographic printers are required to print images on many types of print media. In addition to printing on standard 81/2".times.11" paper, the printers must print images on print media which is non-standard in length, width, thickness and/or weight. One common printing application is printing text or graphics on a continuous roll of labels. The roll of labels is narrower than standard paper, and has varying thickness due to the thick labels being spaced apart on the thin backing paper. Due to variations in size and weight of print media, each print media has its own optimum velocity at which it should be moved through the electrophotographic printer.
Prior art electrophotographic printers print satisfactorily on standard print media, but often print poorly on non-standard print media. Part of the problem is that the printers are designed to move the media at a constant velocity regardless of the size and weight of that media. Thus, when non-standard print media is used, the printers move the media at the same velocity as a standard print media, rather than moving the .media at a velocity appropriate for that particular print media. Using the above example of 300 scan lines per inch, the printers may, for example, move the standard print media at 300 scan lines per inch, but only move the non-standard print media at 295 scan lines per inch. This error of five scan lines per inch cumulates over each sheet of print media. For a standard 81/2".times.11" paper, the total error will be 55 scan lines (i.e., 5 scan lines per inch.times.11 inches). This error is further compounded over a series of sheets in continuous-form printing.
Apart from media considerations, printers naturally age and wear over time. As a result, the media velocity and/or the scan rate change over time which also introduces error into the synchronization of media velocity and scan rate. Additionally, variations in manufacturing tolerances in new parts may introduce error. The cumulative effect of the errors introduced by media differences, printer wear, and variations in manufacturing tolerances may result in poor print quality.
An advantage of the present invention is to provide an electrophotographic printer which adjusts the print media velocity when each new print media is initially loaded into the printer. In this manner, the desired velocity is regularly measured and updated to compensate for any error introduced by change in print media (size or weight), printer age and wear, or variations in manufacturing tolerances.
These and other advantages of this invention will become apparent upon reading the following detailed description of a preferred embodiment.